The invention is based on a hydraulic control device as defined hereinafter. In hydraulic control devices that operate with closed fluid-filled chambers and in which pistons and valve faces are acted upon, the basic relationship between the fluid-impinged surface area and the forces derivable from the fluid pressures and surface areas always presents problems when the time factor plays a role. The same force for instance can be attained with a low pressure and a large surface area or conversely with a high pressure and a small surface area; but for the time factor, it is the control of the volume or in other words the quantity flowing out per unit of time that is definitive. The valves used for control should have an extremely short opening and closing time factor, on the one hand, and on the other should have as little flow resistance, or in other words as small a control cross section, as possible. A hydraulic control device of this kind becomes problematic if large volumes of fluid at high pressure must be controlled at very short time intervals, as is the case for instance with fuel injection, when the control must be effected in less than a millisecond and must vary with the rpm.
In a known fuel injection pump (German Offenlegungsschrift 29 25 418.0), the fuel injection can be interrupted by relieving the pressure of the pump work chamber via a relief conduit; this conduit is controlled via a magnetic valve. The movable valve element of the magnetic valve is acted upon directly by the high pressure of the pump work chamber, so when the valve is closed, if no outflow is to take place, then depending on the functional surface area operative in the opening direction of the movable valve element, considerable forces must be brought to bear to keep the valve closed. With a magnetic valve that is open when there is no current through it, which for safety reasons is the kind of valve usually used for this purpose, the energy consumption of the magnet is proportional to the forces required to keep the valve closed. To obtain the opening time cross section, which is a product of the opening time and the opening cross section, that is required for control, a relatively large cross section must usually be selected, given the short times that are available; this means large, expensive magnets and a correspondingly high consumption of electricity.
In another known fuel injection pump (German Offenlegungsschrift 36 22 627), a pressure line branches off from the pump work chamber, and the fuel is supplied via this line to a deflecting piston, which is displaced by the feed pressure of the injection pump whenever the magnetic valve is open; the space on the back side of the deflecting piston is pressure-relieved by the magnetic valve. This piston is loaded by a restoring spring, which piston injects this fuel received as it deflects into the combustion chamber of the engine via the injection nozzle toward the end of the injection and after the end of the high-pressure feeding. The result is a longer injection duration and quieter engine idling. In this pump, once again, the movable valve element of the magnetic valve is virtually at the feed pressure of the injection pump, because this very high pressure is lessened only by the force of the restoring spring. In other words, once again high closing forces must be brought to bear, entailing correspondingly high cost and requiring a large amount of space and energy.
This is in principle also true if a mechanically actuated adjuster is used, instead of a movable valve element of a magnetic valve.